dchen@oliveb.UUCP (06/25/87)
Why did my C compiler(came with 4.3bsd) complaint the "syntax error"
with the following COMMA separated express statements in a function:
funct()
{
int i = 0;
if ( i == 0 )
printf("A\n"), return; /* syntax error here */
printf("END\n");
}
But it works fine with
if ( i == 0 )
printf("A\n"), exit(1); /* compile OK */
Could someone point this out ? It seems to me that rule 7.15 Comma
operator, on page 192 of C Programming Language doesn't state much
on this.
Dennis Chen
{allegra, hplabs, idi, ihnp4, sun}!oliveb!dchenguy%gorodish@Sun.COM (Guy Harris) (06/25/87)
> Why did my C compiler(came with 4.3bsd) complaint the "syntax error" > with the following COMMA separated express statements in a function: ... > printf("A\n"), return; /* syntax error here */ > > Could someone point this out ? It seems to me that rule 7.15 Comma > operator, on page 192 of C Programming Language doesn't state much > on this. Yes, it does. It states that a "comma-expression" is an "expression", a comma, and another "expression". "return", however, is NOT an "expression", it's a "statement". See p. 203. Guy Harris {ihnp4, decvax, seismo, decwrl, ...}!sun!guy guy@sun.com
ron@topaz.rutgers.edu (Ron Natalie) (06/25/87)
The syntax of comma operator is expr , expr RETURN is a control keyword not an expression. You can't say things like x = return; Exit is a function (it never returns, but C doesn't know that generally). Functions are a simple form of expression, hence you can say x = exit(19) or use it in either operand of a comman operator. -Ron
nagel@iris.ucdavis.edu (Mark Nagel) (06/25/87)
In article <1601@oliveb.UUCP> dchen@oliveb.UUCP (Dennis T. Chen) writes: >Why did my C compiler(came with 4.3bsd) complaint the "syntax error" >with the following COMMA separated express statements in a function: > >funct() >{ > int i = 0; > > if ( i == 0 ) > printf("A\n"), return; /* syntax error here */ > printf("END\n"); >} > >But it works fine with > > if ( i == 0 ) > printf("A\n"), exit(1); /* compile OK */ > >Could someone point this out ? It seems to me that rule 7.15 Comma >operator, on page 192 of C Programming Language doesn't state much >on this. > >Dennis Chen The first example has an expression (a function) comma separated with a statement. The comma operator is supposed to evaluate to the rightmost expression. Since the rightmost item here is not an expression, you get the problem. The second example illustrates this -- exit is a function, thus an expression. --- Mark Nagel >>> nagel@iris.ucdavis.edu ARPA >>> mdnagel@ucdavis BITNET >>> ..!{ucbvax,sdcsvax,lll-crg}!ucdavis!iris!nagel UUCP
michaud@decvax.UUCP (Jeff Michaud) (06/26/87)
> foo() > { > if( x == 0 ) > func(), return; > } The compiler should complain about the following because the "," operator works with expressions, and "return" is NOT an expression (ie. it doesn't have a value). > ... > func(), exit(); > ... is OK because "exit()" is a function call and therefore has a value. (though I am willing to guess that it would be invalid if "exit()" was defined as a function returning "void". Jeff Michaud michaud@decvax.dec.com michaud@decvax.UUCP Disclaimer....
smvorkoetter@watmum.UUCP (06/26/87)
In article <1601@oliveb.UUCP> dchen@oliveb.UUCP (Dennis T. Chen) writes:
]Why did my C compiler(came with 4.3bsd) complaint the "syntax error"
]with the following COMMA separated express statements in a function:
]
]funct()
]{
] int i = 0;
]
] if ( i == 0 )
] printf("A\n"), return; /* syntax error here */
] printf("END\n");
]}
]
]But it works fine with
]
] if ( i == 0 )
] printf("A\n"), exit(1); /* compile OK */
]
]Could someone point this out ? It seems to me that rule 7.15 Comma
]operator, on page 192 of C Programming Language doesn't state much
]on this.
]
]Dennis Chen
]{allegra, hplabs, idi, ihnp4, sun}!oliveb!dchen
`return' is not an expression, it is a statement. `exit' on the other
hand is a function (which never returns, but that is a different story),
and is thus an expression.crowl@rochester.arpa (Lawrence Crowl) (06/30/87)
In article <1601@oliveb.UUCP> dchen@oliveb.UUCP (Dennis T. Chen) writes: >Why did my C compiler(came with 4.3bsd) complaint the "syntax error" ... Dozens of people respond: >Because `return' is a statement and not an expression. PLEASE MAIL RESPONSES LIKE THIS. There must be a thousand readers of this newsgroup that can answer questions like this. If everyone posted an answer to an individual instead of replying through mail, this news group would be useless. Post only that which is of general interest to everyone. The answers to this type of question are of interest to those that asked, and few others. -- Lawrence Crowl 716-275-5766 University of Rochester crowl@rochester.arpa Computer Science Department ...!{allegra,decvax,seismo}!rochester!crowl Rochester, New York, 14627
guy%gorodish@Sun.COM (Guy Harris) (06/30/87)
> There must be a thousand readers of this newsgroup that can answer questions > like this. Unfortunately, there may also be a thousand readers of this newsgroup who might *ask* questions like this; if posting rather than mailing the answer can give somebody else the answer *before* they ask the question, then it may be worth it. Guy Harris {ihnp4, decvax, seismo, decwrl, ...}!sun!guy guy@sun.com